1. Technical Field
This invention relates to the art of RF amplifiers and, more particularly, to an RF amplifier having a dual slope phase modulator for varying the phase of an RF signal.
2. Description of the Prior Art
It has been observed in an AM transmitter that the RF carrier signal may become shifted in phase because of variations in the level of audio modulation. This may result in distortions particularly in digital radio broadcasting. It is an objective to provide RF phase predistortion to compensate for any phase error caused by an amplitude modulation system. This should be accomplished without changing the duty cycle (mark to space ratio) of the output RF signal as compared with the input RF signal.
It has been known in the prior art to phase shift an RF signal. One example of a phase shifting circuit is illustrated in FIG. 1 to which attention is now directed. In this version, an RF signal made up of a train of pulses 10 is obtained from a suitable RF source 12. This RF signal is supplied by way of a phase shifting circuit to an RF transmitter, including a power amplifier 20, and, thence, to an antenna 22. The phase shifting circuit includes a potentiometer including a resistor portion 30 connected between ground and a B+ voltage supply source and an adjustable wiper arm 32 which applies a control level signal, such as V.sub.c, by way of an inductor 34 to a junction point 36. This junction point is fed with the RF signal by way of a capacitor 40 and a resistor R1. A tuning diode CR1 is connected between ground and the junction point 36. The junction point 36 is connected to the power amplifier 20 by way of a capacitor 42. The capacitor 42 is connected by a resistor 44 to ground. In this version of the prior art, the slopes of the input RF signal 10 are modulated to create a small amount of phase shift. The modified RF output signal is digitized and transformed back to a logic level. Consequently, a simulated phase shift is created.
The tuning diode CR1 is the heart of the circuit and its body capacitance changes proportionally to the voltage applied across it. A variable low pass filter comprised of resistor R1 and diode CR1 is created by varying the control signal V.sub.c. This variable low pass filter modifies the slope of the incoming RF signal to create a virtual phase shift by tilting the slopes of the rising and falling edges to obtain the RF output signal as shown at 50. The control is limited because the control level (V.sub.c) is filtered through a low pass filter which creates group delay and limited bandwidth.
Reference is now made to FIG. 2 which illustrates another prior art circuit for use in shifting the phase of an RF signal. This circuit is similar to that of FIG. 1 and to simplify the description herein similar components are identified in both figures with the same character references and only the differences will be discussed below.
In FIG. 2, the phase control shifting circuit includes a differential amplifier 60 including a pair of NPN transistors 62 and 64 having their emitters connected together in common and, thence, through a resistor 66 to the wiper arm 32 of the potentiometer. The base of transistor 62 is connected to the junction of a resistor 70 and a capacitor C1 connected together in series between ground and the collector of the transistor. The base of transistor 64 is connected to ground by a resistor 76 and the collector is connected to a V+ voltage source. The collector of the transistor 62 is connected to resistor R1 and by way of a capacitor 42 it is also connected to the input of the power amplifier 20. The capacitor 42 is connected to ground by way of resistor 74. The control voltage V.sub.c obtained from the potentiometer is used to control the current gain of the transistors and, in turn, the current flow through the capacitor C1. By varying the current through capacitor C1, a variable low pass filter is created comprised of resistor R1 and capacitor C1. This variable low pass filter modifies the slope of the incoming RF signal and creates a virtual phase shift by tilting the slopes of the rising and falling edges of the RF output signal 50.
The output duty cycle (mark to space ratio) in the circuits of FIGS. 1 and 2 is not directly proportional to the input signal because the RF output signal is AC-coupled and this removes the DC information on the original RF input signal causing a level shift of the RF output signal.
Also, the circuits shown in FIGS. 1 and 2 have very limited dynamic range of adjustments because while the slope of the signal is gradually changed, the envelop of the signal is also gradually changed. The acceptable range of phase modulation is on the order of one tenth of an RF wavelength.